Method of manufacturing a light valve

ABSTRACT

Light valve for use in the projection of an image in which a liquid crystal is addressed by an electron beam produced in a column (10). The vacuum interface of the light valve (12) consists of an electrically inhomogenous structure (54) of fine conductors arranged within an insulating matrix. The invention is concerned with the method of manufacturing the light valve vacuum interface to permit high resolution to be obtained, the method comprising filling holes in a glass capillary array, as currently used for filtration of liquids with a molten metal or alloy, and allowing the liquid to cool. The filling is effected by placing the glass capillary array in an evacuated chamber and forcing the molten alloy into the holes in the array under pressure.

The present invention relates to a method of manufacturing anelectronically addressable light valve, which term is intended to mean adevice the optical properties of which may be altered by an incidentelectron beam.

A light valve is generally a device which gates or modulates light. Manymaterials which are capable of having their optical propertiesdynamically altered have been used as a light valves and such valveshave been constructed based on various effects includingacousto-optical, electro-optical, thermo-optical and ferroelectriceffects.

Although several light valve projection systems have been investigated,only a few have reached maturity and gained market acceptance, theoldest and most notable being the Eidophor. In terms of providing auseful display, typical light valve projection systems have sufferedfrom several drawbacks including low resolution, low contrast, limitedlife time and cost.

The invention is particularly concerned with liquid crystal lightvalves. Liquid crystals are known to have their optical propertiesaffected when exposed to an electric field and attempts have been madeat addressing liquid crystals by means of an electron beam.

In constructing a light valve based upon a liquid crystal material, itis necessary for the liquid crystal material to be hermetically isolatedfrom the evacuated space of the electron beam and it is thereforenecessary to construct an interface which permits the liquid crystal tobe scanned by the electron beam while containing the vacuum.

If the interface is an insulator and capacitative coupling is reliedupon to alter the potential on the liquid crystal, then there occurscharge spreading which reduces the resolution of the image.

In the IBM Technical Disclosure Bulletin, Vol. 16., No. 1, pp. 353,354,there is disclosed an interface between the liquid crystal material andthe electron beam apparatus which comprises a two dimensional array ofconducting pins in a dielectric substrate. This avoids the problem ofcharge spreading but the resolution of the image is limited by theseparation of the conducting pins.

Various other applications have also required an interface with a beamwhich comprises conductive pins in an insulating substrate. Examples ofthese are described in GB-A-2 029 633 which is concerned with anelectrostatic printing tube and GB-P-1 509 823 which relates toelectron-radiography apparatus.

The present invention seeks to enable the manufacture of an interfacebetween an electron beam apparatus and a liquid crystal material whichis capable of providing an image of high resolution.

According to the present invention, there is provided a method ofmanufacturing an interface between a liquid crystal and the evacuatedspace of an electron beam apparatus, which interface comprises aplurality of conductive portions which extend across the thickness of aninsulating substrate and which are electrically isolated from oneanother, the method being characterised by the steps of placing a glasssubstrate having a two dimensional array of holes of the desired sizeand separation formed therein within a chamber, firmly supporting theglass substrate against a backing plate, placing a solid block of alloywithin the chamber, evacuating the chamber, applying heat to melt thealloy and urging a piston movable within the chamber towards the glasssubstrate to force the molten alloy into the evacuated holes in theglass substrate.

The molten alloy does not flow into the holes in the glass substratewith ease on account of surface tension effects. However, by firstevacuating the holes and forcing the molten alloy into the holes underpressure, it is possible to fill the holes and when the alloy solidifiesa plurality of conductive pins are left within the substrate which alsorender the interface vacuum tight so that it may form part of the vacuumenvelope of an electron beam apparatus.

Glass substrates suitable for the manufacture of the interface arecommercially available and are used as fluid filters. They are believedto be manufactured by pulling glass fibres of one type of glass througha second type of glass which has different etching properties. Severalcoated fibres are fused together and then etched. The etching leavesfine holes in the positions of the original fibres. By this method, itis possible to form holes which are 10 microns, or less, apart but thestructure is rather fragile. The invention permits such a substrate tobe filled with a conductive alloy and the risk of breakage of thesubstrate as a result of the applied pressure is reduced by the use ofthe backing plate, which should match the contours of the substrateclosely. The glass and the backing plate will normally be ground flat.

It is preferred that the chamber be evacuated while the solid alloyrests against the glass substrate. To assist in evacuating the holes inthe glass substrate holes are preferably formed in the solid alloybefore it is molten and heat is applied to the alloy only after thechamber has been evacuated.

Conveniently, the alloy may be melted by means of a resistive heaterdisposed within the piston. Alternative other means, such as inductionheating, may be employed for heating the alloy within the evacuatedchamber.

It is important to be able to control the heating and cooling rate ofthe alloy. If the chamber is made of a good thermal conductor, it isdifficult to control the temperature with accuracy in view of thesignificant heat losses through the chamber walls. It is thereforepreferred that the substrate and the alloy be confined within ainsulating block within the chamber to restrict the heat losses.

It may prove advantageous to provide a further electrode adjacent butelectrically isolated from the array of pins, for example to prevent abuild up of charge on the insulating matrix.

To enable such an electrode to be formed on the interface, the embeddedelectrically conductive portions or pins may be etched so that theirends lie beneath the surface of the insulating matrix and anelectrically conductive layer may then be formed on the surface, theresulting electrode thereby being spaced from the ends of the embeddedpins.

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which;

FIG. 1 shows a light valve projection system,

FIG. 2 shows a section through a light valve, drawn to an enlargedscale, and

FIG. 3 shows schematically a rig for manufacturing the interface.

In FIG. 1, there is shown a light valve projection system in which theimage on the liquid crystal is viewed by reflection. The systemcomprises an electron beam column, generally designated 10, which scansa light valve 12. The light valve 12 is viewed from the opposite side tothe electron beam by means of a light source which illuminates theliquid crystal by way of illumination optics and a dielectric mirror 60.The light reflected by the liquid crystal is again reflected by thedielectric mirror towards projection optics which focus the light on aviewing screen. A light absorber 62 reduces undesired reflections.

Though the preferred system described above views the light valve byreflection, it is alternatively possible for the image to be viewed bytransmission.

The electron beam column 10 is generally similar to that used in anelectron microscope and is itself known. The device comprises anelectron gun 22 and an anode stage 24. The electron beam passes througha first spray aperture 26 into a first focussing lens 28 and then passesthrough a second spray aperture 30 into a second lens 32. The sprayapertures cut out any scattered electrons in order to maintain contrastand image purity. Also arranged within the path of the electron beam isa stigmator 34, a vacuum isolation valve 36, a probe forming lens 38 andhigh resolution deflection coils 40. The stigmator corrects anyastigmatism in the beam.

The electron beam column described above is complex because of the factthat is has been designed as part of a test apparatus to permit veryconsiderable control over the focussing, scanning and modulation of theelectron beam. In any practical embodiment, the electron beam column maybe considerably simplified.

If the image is viewed by transmission, it may be necessary to arrangethe plane of the light valve at an angle to the electron beam axis. Thebeam column described above is also capable of dynamic focussing of thebeam to take into account the relative inclination of the light valvebut once again simplification is possible when the light valve is normalto the beam axis, as is the case in the described preferred embodiment.

The light valve 12 is shown in more detail in FIG. 2 and comprises aliquid crystal material 50 sandwiched between a glass plate 52 and aliquid/vacuum interface 54. The liquid crystal material may convenientlycomprise a smectic A liquid crystal or a dyed phase change guest hostliquid crystal with the guest being a nematic liquid crystal.

The interface 54 is comprised of a glass capillary array 56 having veryclosely spaced holes of very small diameter. The holes within the glasscapillary array 56 are filled with an electrically conductive material,for example an indium or other suitable alloy to form an array ofclosely spaced conductive pins 58. After the conductive material hasfilled the holes in the capillary array 56, it is etched away, asillustrated, by an suitable means, so that the surface of the pins isrecessed from the surface of the glass capillary array 56. A furtherelectrode 66 for removing any electrostatic charge on the capillaryarray 56 is then deposited without danger of short circuiting the pins58.

The glass capillary array may be such as manufactured by GalileoElectro-optics Corporation of Massachussetts and used for the filtrationof liquids. The holes in such a capillary array permit the manufactureof an interface with high resolution. Typically, the distance betweenpins is less than 13 microns and may be as little as 3 microns. Thematerial used to fill the holes may consists of a metal or an alloycontaining indium, tin, copper, lead, cadmium, bismuth and/or gallium.

In order to fill the holes, the glass capillary array is placed above alayer of the alloy in an evacuated piston cylinder apparatus. The alloyis heated to melting point and pressure is then applied to force themolten alloy into the holes of the capillary array. A rig for carryingout the above method is shown schematically in FIG. 3.

In FIG. 3, there is shown a metal enclosure 70 defining a chamber 72. Apipe 74 leads from the chamber 72 to a vacuum pump for evacuating thechamber 72. The upper end of the enclosure is formed of a rigid plate 76secured to the side walls 78 of the enclosure and capable ofwithstanding high pressure without being deflected to any materialextent.

A hollow piston 80 is slidably mounted within the chamber 72 and asuitable drive is provided (not shown) for moving the piston 80 upwardstowards the plate 76. The drive may either be a manually adjustablescrew or a hydraulic cylinder. A heater 82 is disposed within the piston80 for melting the alloy which is to be forced into the holes in theglass capillary array and a thermocouple 84 permits the temperature tobe monitored.

A block 90 made in several parts from a thermally insulating material,such as P.T.F.E., is provided within the chamber 72 to define anenclosure within which the alloy is melted. The block 90 is only shownschematically and its purpose is to confine the molten alloy to a smallvolume within which the temperature may be controlled accurately and toenable the interface to be extracted simply from the rig.

Within the block 90, there is disposed a back plate 92 against whichrests the glass substrate 56. The alloy 96 to be melted is also placedwithin the block 90 and a spacer 98 ensures a small gap for evacuationof the holes in the glass substrate. Holes are provided in the alloy forthe same purpose.

In use, the chamber 72 is evacuated with the parts held in the positionillustrated. The piston is then moved upwards into the block 90 andpasses in the process through an O-ring 99 which seals against the outersurface of the piston 80. The piston is stopped when it reaches thealloy 96 and the heater 82 is energised to melt the alloy. When thealloy is molten, the piston is moved up further to force the moltenalloy into the holes of the glass substrate 56. As the space haspreviously been evacuated, the molten alloy can flow to fill the entirevolume within the block 90.

The pressure which must be applied is quite substantial and would breakan unsupported glass substrate. However, the backplate 92 disposed theblock 90 prevents deflection and damage. The backplate 92 is separatedfrom the plate 76 by a section of the block 90 to prevent thermallosses.

The heater is now switched off and the temperature reduced at acontrolled rate until the alloy solidifies. At this time, the block 90may be withdrawn from the enclosure and separated to allow removal ofthe glass substrate with the adhering alloy. The surplus alloy ismachined away to leave the glass capillary array 56 complete with metalpins.

The pins may now be partially etched, as earlier described, and thesurface ot the glass coated with a further electrode for the removal ofelectrostatic charge building up on the glass.

Though the system may be used for projection television, it is furtherpossible to use the projection system to produce large area displays ofvery fine resolution as may be required, for example, in the study oflarge scale integrated circuits.

We claim:
 1. A method of manufacturing an interface between a liquidcrystal and the evacuated space of an electron beam apparatus, whichinterface comprises a plurality of conductive portions which extendacross the thickness of an insulating substrate and which areelectrically isolated from one another, the method comprising the stepsof placing a glass substrate having a two dimensional array of holes ofthe desired size and separation formed therein within a chamber, firmlysupporting the glass substrate against a backing plate, placing a solidblock of metal within the chamber, evacuating the chamber, applying heatto melt the metal, urging a piston movable within the chamber towardsthe glass substrate to force the molten metal into the evacuated holesin the glass substrate and allowing the molten metal to cool, wherebymetallic pins are formed in the holes in the glass substrate.
 2. Amethod as claimed in claim 1, wherein the chamber is evacuated while thesolid block of metal rests adjacent the glass substrate, holes beingprovided in the solid block of metal to permit evacuation of the holesin the glass substrate.
 3. A method as claimed in claim 1, wherein thealloy is heated by means of a resistive heater disposed within thepiston.
 4. A method as claimed in claim 1, wherein metal is heatedinductively.
 5. A method as claimed in claim 1, wherein the substrateand the metal are confined within a thermally insulating block withinthe chamber to restrict the heat losses and to permit extraction of theinterface from the chamber.
 6. A method as claimed in claim 1,comprising machining and grinding at least one surface of the glasssubstrate in order to remove surplus metal from that surface.
 7. Amethod as claimed in claim 1, comprising etching the metallic pins sothat their ends lie beneath at least one surface of the glass substrate,and forming an electrically conductive layer on said one surface.
 8. Amethod as claimed in claim 1, wherein the distance between the centersof adjacent pins is less than 13 μm.
 9. A method of manufacturing anarticle comprising a plate-form member of dielectric material and aplurality of conductive portions which extend across the thickness ofthe plate-form member and which are electrically isolated from oneanother, the method comprising the steps of placing a glass substratehaving a two dimensional array of holes of the desired size andseparation formed therein within a chamber, firmly supporting the glasssubstrate against a backing plate, placing a solid block of metal withinthe chamber, evacuating the chamber, applying heat to melt the metal,urging a piston movable within the chamber towards the glass substrateto force the molten metal into the evacuated holes in the glasssubstrate and allowing the molten metal to cool, whereby metallic pinsare formed in the holes in the glass substrate.